116 



GENERAL PRINCIPLES OF ZOOLOGY 



above or in front of the mouth. In the air-breathing vertebrates this 

 pair of pits, which here also arise from the skin, are taken into the dorsal 

 wall of the two respiratory canals leading from the outside to the mouth 

 or pharynx. Now since the olfactory cells distributed in these pits (fig 

 38, B) are frequently characterized by bundles of olfactory hairs, while 

 the surrounding epithelium is often ciliated, one is inclined to regard as 

 organs of smell sensory organs of invertebrates, which have the form of 

 ciliated pits or lie near the respiratory apparatus (e.g., the osphradiwn of 

 molluscs). Yet there are exceptions. Experiments seem to show that in 

 the arthropods the antennae serve for smelling. Here the sensory per- 

 ception can be connected only with certain modified hairs, the olfactory 

 tubules of the Crustacea and the olfactory cones of insects. In a similar 

 way certain nerve end organs in the region of the mouth are considered 

 as organs of taste, since the taste organs of vertebrates, the so-called taste 

 buds, are abundant in the mouth cavity. 



Organs of Hearing and of Sight are called the higher sense-organs, 

 because they are of much greater importance than the other organs, 



Ot 



N 



Wz 



FIG. 82. Auditory vesicle of a mollusc (Pterotrachea). N, auditory nerve; Us, audi- 

 tory cells with the central cell; Cz, Wz, ciliated cells, Ot, otolith (after Claus). 



since they furnish sensations which are quantitatively and qualitatively 

 much more definite. Ears and eyes have therefore a complicated and 

 characteristic structure, which renders them easily recognizable by the 

 almost invariable presence of certain structures accessory to their 

 functions. 



The auditory organs of vertebrates and of most other animal groups 

 can be traced back to a simple fundamental form, the auditory veside 

 (fig. 82). This has an epithelial wall, a fluid contents, the endolymph, 

 and an auditory ossicle or otolith, formed from one or from several 

 lused concretions. In some instances the otoliths, to the number of 



